ST9905-CS050913 (2)September 13, 2005
Mr. Ken Griffin, P.E.
City of Coppell
255 Parkway
Coppell, Texas 75019
Bethel Road 1
Pacheco Koch/Duke Construction Drainage Request
,~[L[,L~_ _ ~--~_~.~2Z ~i ,_ - Duke Freeport Addition
Dear Mr. Griffin:
This letter is a follow-up to the September 8, 2005 meeting at the City of Coppell with Pacheco
Koch, Duke Construction, TranSystems and the City of Coppell. To recap the meeting, Pacheco
Koch and Duke Construction are requesting permission to discharge storm water from the south
parking lots of two buildings (Freeport XIV & XV, Duke Freeport Addition) into the proposed
drainage system for Bethel Road.
The proposed drainage system (Line D) includes 720 LF of 21" RCP, 300 LF of 24" RCP and 57
LF of 27" RCP which discharges into one of 4-10'x5' box culverts. The design discharge of the
27" RCP is 20.72 cfs, with a capacity of 21.89 cfs. The total proposed discharge from the south
parking lots of the Freeport XIV & XV buildings is 18.41 cfs, with two thirds of the discharge
entering the 24" RCP. Therefore, the entire length of proposed Line D would need to be up sized.
Other than the cost for redesign and upsizing the RCP, which would be bore by Duke
Construction, upsizing the RCP within Bethel Road is not an issue.
The real issue is the capacity of the 4-10'x5' box culverts, as explained in the December 13, 2002
letter supplement to the May 30, 2002 drainage report:
Currently, the selected alternate provides for the construction of an additional 3-10'X 5' box
culvert beginning at the outfall from the USPS open channel and extending 300' downstream of
Freeport Parkway, eventually outfalling to an improved open channel section. Our analysis
shows that if the airport does not limit discharges to match existing conditions the proposed
storm water system would no longer pass the lOO year discharge. The table below is provided to
summarize our analysis.
3010 LBJ Freeway, Suite 990 · Dallas, Texas 75234 · Phone: (972) 280-9300 Fax: (972) 280-9715
Table-l: Fully-Developed Discharges at Various Locations Along Stream G-1
No Detention on Airport Property
Location Drainage 100 Year 50 Year 25 Year 10 Year 5 Year
Area
Upstream of 464 2515 2133 1901 1550 1289
Bethel Rd.
Upstream of 406 2437 2062 1835 1492 1239
Freeport Pkwy.
Upstream of 242 1328 1126 1006 828 694
Royal Lane
Table 1 summarizes the discharges for a fully developed basin with no detention provided on the
airport property. The 4-10' X 5' box culverts (3-proposed and one existing) have a capacity of
1690 cfs before overtopping occurs and street flooding begins. This translates to a protection
level for the 25-year storm assuming no detention is provided on fully developed airport
property.
In sununary, TranSystems sees no apparent problem upsizing the RCP within Bethel Road to
accommodate storm drain from the Freeport XIV & XV buildings. The 4-10'x5' box culverts
have a 25-year capacity of 1872 cfs (upstream of Freeport), so an additional 18.41 cfs added to
the 1835 cfs listed above does not exceed the 25-year capacity. However, since no detention was
provided on Airport property, the system will still not be capable of handling the 100 year storm
event. Please let us know how you would like us to proceed.
Sincerely,
TranSystems Corporation Consultants, Inc.
Project Manager
CC: Garry Kraus, P.E. - TranSystems
G:~00\00890-Coppell'xHydraulics~Pacheco Koch.doc